U.S. patent application number 12/188870 was filed with the patent office on 2009-02-12 for electron microscope.
This patent application is currently assigned to HITACHI HIGH-TECHNOLOGIES CORPORATION. Invention is credited to Akira IKEGAMI, Hideyuki Kazumi, Hisaya Murakoshi, Koichiro Takeuchi, Minoru Yamazaki.
Application Number | 20090039264 12/188870 |
Document ID | / |
Family ID | 40345589 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090039264 |
Kind Code |
A1 |
IKEGAMI; Akira ; et
al. |
February 12, 2009 |
ELECTRON MICROSCOPE
Abstract
Disclosed herein are a method for applying, while a charged
particle beam is in a state being irradiated toward the sample, a
voltage to the sample so that the charged particle beam does not
reach the sample (hereafter such state may be referred to as a
mirror state) and detecting information on a potential of a sample
using a signal obtained then, and a device for automatically
adjusting conditions of the device based on the result of
measuring.
Inventors: |
IKEGAMI; Akira; (Suita,
JP) ; Yamazaki; Minoru; (Hitachinaka, JP) ;
Kazumi; Hideyuki; (Hitachinaka, JP) ; Takeuchi;
Koichiro; (Hitachinaka, JP) ; Murakoshi; Hisaya;
(Tokyo, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
HITACHI HIGH-TECHNOLOGIES
CORPORATION
|
Family ID: |
40345589 |
Appl. No.: |
12/188870 |
Filed: |
August 8, 2008 |
Current U.S.
Class: |
250/311 |
Current CPC
Class: |
H01J 2237/28 20130101;
H01J 37/28 20130101; H01J 37/21 20130101; H01J 2237/24564 20130101;
H01J 2237/30461 20130101; H01J 37/3045 20130101; H01J 2237/2594
20130101; H01J 2237/2816 20130101; H01J 37/244 20130101 |
Class at
Publication: |
250/311 |
International
Class: |
G01N 23/00 20060101
G01N023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2007 |
JP |
2007-207342 |
Claims
1. A scanning electron microscope comprising a control device for
adjusting a negative voltage applied to a sample to control energy
of an electron beam reaching the sample, wherein; the control
device: comprises a storage medium for storing a curve showing
variation of information obtained based on the reflection electron
when, under the state the negative applied voltage is adjusted so
that the electron beam reflects without reaching the sample by an
electric field formed by the negative applied voltage, the negative
voltage is varied, and; performs detecting a potential of the
sample, controlling lens strength of an objective lens for
converging the electron beam, or controlling a scanning deflector
for scanning the electron beam based on a shift amount between the
curve stored in the storage medium and a new curve obtained when
the negative voltage is adjusted.
2. The scanning electron microscope as set forth in claim 1,
wherein, in applying a voltage to the sample, the voltage applied
to the sample is adjusted based on the measured potential of the
sample.
3. A scanning electron microscope comprising a control device for
adjusting a negative voltage applied to a sample to control energy
of an electron beam reaching the sample, wherein; a detector for
detecting the reflected electron is disposed at a crossover plane
of the reflected electron formed when the negative applied voltage
is adjusted by the control device so that the electron beam
reflects without reaching the sample by an electric field formed
based on the negative applied voltage.
4. The scanning electron microscope as set forth in claim 3,
wherein the detector is disposed between the deflector for scanning
the electron beam and the objective lens.
5. The scanning electron microscope as set forth in claim 4,
wherein the detector is constituted to allow a plurality of
detecting elements to be arranged two-dimensionally.
6. The scanning electron-microscope as set forth in claim 4,
wherein the control device controls the objective lens to allow an
open angle of the electron beam to vary in measuring the potential
of the sample.
7. The scanning electron microscope as set forth in claim 4,
wherein the control device makes a deflection fulcrum of the
deflector vary synchronizing with variation in the negative voltage
applied to the sample in measuring the potential of the sample.
8. A scanning electron microscope comprising a control device for
adjusting a negative voltage applied to a sample to control energy
of an electron beam reaching the sample, wherein, when a potential
gradient has been generated in a scanning region of the electron
beam, the control device forms charging in the scanning region by
scanning of the electron beam.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The purpose of the present invention is to provide a charged
particle beam device suitable for reducing focus offset,
magnification fluctuation and measurement length error in the
charged particle beam device caused by charging on a sample.
[0003] 2. Description of the Related Art
[0004] Recently, as semi-conductor devices, particularly, progress,
measuring and inspecting technique of a semi-conductor is more and
more increasing its importance. A scanning electron microscope
represented by a CD-SEM (Critical Dimension-Scanning Electron
Microscope) is a device for measuring the pattern formed on a
semi-conductor device by scanning an electron beam on a sample and
detecting electrons such as secondary electrons or the like emitted
from the sample. In such a device, although the condition of the
device is required to be appropriately set to carry out highly
accurate measurement and inspection, among recent devices, there
are samples wherein the charge adheres by irradiation of an
electron beam or influence of a semi-conductor process. Insulator
samples such as resist, insulating film, low-k material and the
like, in particular, are known as the samples to which the charge
is liable to adhere.
[0005] Following methods are conventionally devised as methods for
measurement of a charging potential. In Japanese Unexamined Patent
Application Publication (JP-A) No. 7-288096, a method is disclosed
wherein an electron beam is converged on a sample, the electron
beam is scanned on the sample, a "reflecting electron" signal
obtained according to irradiation of the beam is detected by a
detector, an amount of variation of the detected signal in a
predetermined time is determined, and either of the pressure around
the sample, irradiation amount of the electron beam, and the
acceleration voltage of the electron beam is controlled based on
the amount of variation obtained (conventional technique 1). Also,
a controlling method wherein charging is detected and feed-back is
applied based on it is disclosed in the U.S. Pat. No. 6,521,891
(conventional technique 2). According to it, an electron beam is
scanned on a sample, a secondary electron and a backscattering
electron are detected and an image is formed. The image is obtained
by varying accelerating energy of the electron beam, and
accelerating energy of the electron beam is varied based on the
result of analysis of the image, thereby compensation of charging
of the sample is pursued. On the other hand, a method is exhibited
in the gazette for the JP-A-1-214769 wherein non-contact measuring
of a potential of a sample is performed. A metal needle having a
sharpened tip, a feed-back circuit for detecting a field emission
current or a tunnel current through the metal needle and to apply a
voltage to the metal needle so that the current becomes constant,
and a circuit for reading out the metal needle voltage are provided
(conventional technique 3).
[0006] (Patent Document 1) JP-A-7-288096
[0007] (Patent Document 2) U.S. Pat. No. 6,521,891
[0008] (Patent Document 3) JP-A-1-214769
SUMMARY OF THE INVENTION
[0009] Although both of the techniques described in the patent
documents 1 and 2 relate to the technique wherein a charged amount
on a sample is measured and conditions of a device are adjusted
based on the measurement, because the charged amount is measured by
detecting a signal obtained according to irradiation of an electron
beam to the sample, charging is forcibly induced by irradiation of
the electron beam, and a problem that measurement of the charged
amount prior to irradiation of the electron beam is difficult is
involved.
[0010] On the other hand, according to the technique described in
the patent document 3, although measurement of a potential of the
sample surface is possible without inducing charging by an electron
beam, there are problem that it takes time to make the metal needle
get close to a sample, and problems of change of a potential of the
sample by making the metal needle get close to the sample and
discharging in the case charge amount is large.
[0011] The purpose of the present invention is to provide a device
for measuring a potential of charge of a sample adhering by
irradiation of a charged particle beam or influence of a
semi-conductor process without irradiation of an electron beam to
the sample, and automatically compensating the conditions
(magnification, focus, observation coordinates) of the device which
vary according to charge of the sample.
[0012] To achieve the purpose described above, the present
invention provides a device for measuring a potential of a sample
using a signal obtained under a state a voltage is applied to the
sample so that a charged particle beam does not reach the sample
(hereafter such state may be referred to also as a mirror state)
while the charged particle beam is irradiated toward the sample,
and automatically compensating the conditions of the device
(magnification, focus, observation coordinates) which vary
according to charge of the sample.
[0013] A specific method will be described below. The means in
accordance with the present invention consists of two steps
described below.
1. A Step For Measuring a Potential
[0014] Under the mirror state where a primary electron beam is not
incident on a sample, optical parameters (an object point ZC for an
objective lens, exciting current of the objective lens I.sub.obj, a
potential of the sample V.sub.s=V.sub.r+.DELTA.V.sub.s, a potential
of a booster V.sub.b) are set to optional values, and a
displacement amount (the arrival point of orbit H on a detector) or
a magnification (the arrival point of orbit G on the detector) is
measured, thereby the potential of the sample is calculated. The
detecting method of a displacement amount and a magnification will
be exhibited below.
[0015] In measuring a displacement amount and a magnification of a
mirror electron at the detector position, it is preferable to use a
detector with a plurality of detecting elements spreading
two-dimensionally. The arrival position or distribution of the
mirror electron is obtained based on the output signal of the
plurality of detecting elements, thereby it becomes possible to
obtain the deviation from the reference value.
[0016] Further, if a deviation amount is arranged to be detected
using an image, the deviation amount can be detected more easily.
While the mirror electron is reflected right above the sample and
passes through a lens system, it is subjected to influence of a
passage of the beam and a structure. To obtain the image, the
position of the incident beam may be scanned. Thus, the shape of
the structure in the path of the beam is formed as an image. By
measuring the size of the structure shape transferred into the
image and the sag of an edge, the displacement amount and the
magnification can be measured.
[0017] As an example of deriving a potential of a sample from the
displacement amount and the magnification detected as above,
relation between the retarding potential V.sub.r and the
displacement amount is shown in FIG. 2. The curve A in the drawing
was obtained as I.sub.obj was fixed to an optional value
(I.sub.obj1) when the potential by charging .DELTA.V.sub.s=0 V.
When charging occurs in a sample, a potential of a sample V.sub.s
can be represented by a total of a retarding potential V.sub.r and
a potential by charging .DELTA.V.sub.s, therefore the curve of the
displacement amount with respect to the retarding potential shifts
by .DELTA.V.sub.s. Consequently, if the retarding potential V.sub.r
where the displacement amount becomes 0 on the detector is
obtained, the charging potential .DELTA.V.sub.s can be derived by
referring to the displacement amount curve A. Further, by measuring
the displacement amounts at two or more kinds of retarding
potential V.sub.r and estimating the shifting amount of the curve
A, the charging potential .DELTA.V.sub.s can be estimated.
[0018] Here, if the value I.sub.obj is set low (high), the focusing
potential of the sample becomes high (low) and the position Z.sub.m
(mirror surface) where the irradiated electron is reflected becomes
low (high). Therefore, to improve spatial resolution of measurement
of a potential, focus can be adjusted with the value I.sub.obj
being set low and with the mirror surface being made close to the
sample.
[0019] Further, if a potential is measured with the set value of
I.sub.obj changing, measuring of the potential distribution
V.sub.axis on the light axis (Z) is possible as well.
[0020] 2. A step for automatically compensating the conditions of
the device (focus, magnification, observation coordinates) If a
wafer is not charged, exciting current of the objective lens
required for focusing is generally represented by a function shown
in the equation (1).
I.sub.obj=F(V.sub.o, V.sub.r, Z) (1)
where, I.sub.obj is the exciting current of the objective lens when
the wafer is not charged, F is the function for calculating the
exciting current of the objective lens, V.sub.r is the retarding
potential applied to the wafer, and Z is the height of the wafer.
The function F can be derived by optical simulation or actual
measurement. Because the potential of the wafer which is not
charged is generally of the same potential with the retarding
potential applied to the wafer and satisfies the equation (1), the
regular focus control is possible. However, the exciting current of
the objective lens required when the wafer itself is charged is as
shown in the equation (2), therefore, the focus current is
different in the charged case and the non-charged case.
I'.sub.obj=F(V.sub.o, V.sub.s, Z) (2)
V.sub.s is a potential of a sample and can be represented as a
total of a retarding potential V.sub.r applied to a wafer and the
charging potential .DELTA.V.sub.s.
[0021] Consequently, even if the height is detected accurately, the
focus cannot be adjusted, therefore, the secondary charged particle
image is blurred.
[0022] Then, if the potential of the sample
V.sub.s(=V.sub.r+.DELTA.V.sub.s) is measured by the method as
described in 1 and observation is performed using the exciting
current I'.sub.obj obtainable by the equation (2), focus control
becomes possible even when the wafer is charged.
[0023] Although the example described above is the method of
feedback of the result of the measurement of the potential to the
exciting current, focus control by feedback of the potential of the
sample V.sub.s obtained by measurement of the potential to the
retarding potential V.sub.r is also possible.
[0024] Besides, in other case of a SEM using what is called a
boosting method wherein a cylindrical electrode applied with a
positive potential is disposed within an objective lens, focus
control is possible by adjusting the positive potential V.sub.b
applied. Furthermore, other general techniques for adjusting the
focus of an electron beam are applicable.
[0025] The case the present invention is applied to magnification
control of a SEM will be described.
[0026] If the potential of a sample varies by charging, the
magnification of a scanning electron microscope varies. When a
primary electron beam emitted radially from one point of a
crossover plane concentrates to one point of a sample surface, if
an imaginable emitting point of the primary beam shifts by one unit
distance, the imaginable arrival point onto the sample surface
shifts by M.sub.obj unit distance. When the conversion coefficient
and the coil current of a scanning deflector are represented by K
and I.sub.scan respectively, the distance between two points on the
sample can be calculated by the following equation.
A=KM.sub.objI.sub.scan (3)
Also, M.sub.obj can be represented by the following equation.
M.sub.obj=M(V.sub.o, V.sub.r, S.sub.charge, .DELTA.V.sub.s) (4)
where, S.sub.charge is the area of a charged region.
[0027] With regard to magnification variation too, if the function
M.sub.obj has been derived from optical simulation or an
experiment, input current I.sub.scan of a deflecting coil with A
being made constant can be obtained by the equation (3).
[0028] As a third application, image drift control and
magnification control occurring in observation utilizing
preliminary charging can be cited.
[0029] In observing a contact hole with a high aspect ratio, a
phenomenon that the bottom of the contact hole cannot be seen
occurs. Then, a technique is disclosed in the JP-A-5-151927 wherein
positive charging is created on the surface of a sample by
irradiation of an electron beam at a low magnification beforehand,
a secondary electron discharged from the bottom of the contact hole
is elevated by an elevating electric field formed between the
bottom of the contact hole and the surface of the sample, thereby
observation of the bottom of the hole is performed. However,
because of a potential gradient generated in the preliminary
charging, problems such as magnification fluctuation, drift, or the
like occur. An explanatory drawing of the mechanism of occurrence
of drift of an image is exhibited in FIG. 9.
[0030] The charged region (generally, its size is several tens of
.mu.m to several hundreds of .mu.m at one side) formed by the
preliminary charging is shown at A in the drawing. If positive
charge is accumulated evenly over the region, the potential
distribution with the center part being highest as shown in the
lower drawing of FIG. 9 is formed. When observed (one side of the
region of observation is several hundreds of nm to several .mu.m)
after formation of such potential distribution, the primary
electron beam is subjected to a force from the potential gradient
by the preliminary charging and is forcibly deflected. As a result,
the area different from that of the original desire is forcibly
observed.
[0031] If the technique in accordance with the present invention is
used, the distribution of the charging potential can be measured
with high spatial resolution without irradiation of the primary
electron beam onto the sample. Consequently, the present invention
can be applied to measuring the potential gradient of the
observation region prior to observation. If observation coordinates
is compensated based on the result of measurement of the potential
gradient, the desired observation region can be observed. Further,
as is exhibited in FIG. 10, the influence of drift of an image by
charging is inhibited and the bottom of the contact hole with a
high aspect ratio becomes possible to be observed by performing the
preliminary irradiation again to eliminate the potential gradient
of the observation region. This method is effective not only for
observation of the bottom of the contact hole but also for general
technique wherein observation is performed after preliminary
charging.
[0032] According to the constitution described above, observation
of the sample inhibiting the influence of charging by irradiation
of an electron beam is possible, because the potential of the
sample is detected from the information obtained under the state
the charged particle beam is not incident on the sample and optical
conditions are controlled.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Embodiment(s) of the present invention will be described in
detail based on the following figures, wherein:
[0034] FIG. 1 is an explanatory drawing of the outline of a
scanning electron microscope;
[0035] FIG. 2 is an explanatory drawing of a method wherein a
potential of a sample is derived from a displacement amount;
[0036] FIG. 3 is an explanatory drawing of the optical condition
suitable for measuring a potential with high spatial
resolution;
[0037] FIG. 4 is an explanatory drawing of focus offset occurring
when an opening angle increases;
[0038] FIG. 5 is an explanatory drawing of an optical condition
suitable for measurement of a potential in high accuracy;
[0039] FIG. 6 is a flowchart of the first embodiment in accordance
with the present invention (focus control);
[0040] FIG. 7 is a flowchart of the second embodiment in accordance
with the present invention (magnification control);
[0041] FIG. 8 is a flowchart of the third embodiment in accordance
with the present invention (opening angle control);
[0042] FIG. 9 is a flowchart of the fourth embodiment in accordance
with the present invention (deflection fulcrum control); and
[0043] FIG. 10 is an explanatory drawing of an example inhibiting
the potential gradient of the observation region by performing
preliminary irradiation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] The position of a detector appropriate for implementing the
present invention will be described.
[0045] To improve the spatial resolution of measurement of a
potential in accordance with the present invention, two conditions
described below should be satisfied.
[0046] A mirror surface which is a reflecting surface for the
primary electron beam is put near to a sample.
[0047] Spreading of the primary electron on the mirror surface is
minimized.
[0048] FIG. 3 shows an explanatory drawing on an optical condition
(optical condition A) expectable the highest spatial resolution in
measuring a potential utilizing the present invention. In the
drawing, ZC is an object point of an objective lens where a
detector is positioned. If such arrangement as FIG. 3 is employed,
when focused on the detector, focusing is adjusted on the mirror
surface as well. Consequently, in calculating a potential of a
sample from the condition with which focus offset is minimized on
the detector, if the arrangement exhibited in FIG. 3 is employed,
the spatial resolution of measurement of a potential can be
improved.
[0049] Here, a displacement amount reflected to the detector by
variation of the potential of the sample is proportional to an open
angle of the object point (under the case focus offset by
aberration is negligible). Therefore, if the open angle at the
object point is made large, the detection sensitivity of variation
of the potential of the sample improves. However, under the optical
condition as exhibited in FIG. 3, velocity in the lateral direction
is forcibly generated on the mirror surface. Therefore, as the open
angle of the primary beam is larger, the beam is reflected at a
position (A in FIG. 4) which is higher than the mirror surface and
the focus is offset at the detecting surface. Consequently, even if
the open angle is made large and measuring sensitivity of the
potential of the sample is improved, the open angle cannot be made
large because the focus offset attributable to the open angle as
described above occurs.
[0050] To solve the problem described above, an optical condition
(optical condition B) as exhibited in FIG. 5 can be employed. ZC in
the drawing is a crossover plane where a detector is positioned.
Then, the exciting amount of an objective lens is adjusted so that
the inclination of the primary electron beam on the mirror surface
becomes parallel with the light axis. If the electron beam is
irradiated under the condition, any primary electron beam having
any angle at the object point is incident perpendicular to the
mirror surface, reflected at the potential surface of the same
potential, and is converged to the same position on the detector.
Therefore, the sensitivity of measuring the potential can be
improved because the open angle of the primary electron beam used
for measurement can be enlarged. However, because the primary
electron beam is widened spatially at the mirror surface, spatial
resolution deteriorates. Accordingly, if spatial resolution of
measuring a potential is important, the potential can be measured
by the optical condition A, and if measuring accuracy for the
potential is important, the potential can be measured by the
optical condition B. In addition, because the optical condition
optimal for measurement of a potential using a mirror electron
(specifically, crossover position ZC, booster voltage V.sub.b,
retarding voltage V.sub.r, open angle of crossover plane
.alpha..sub.c, and deflection fulcrum Z.sub.p) and the optical
condition optimal for observation do not coincide, it is preferable
to measure switching the optical condition used in measurement of
the potential and in observation.
[0051] In measuring a potential in accordance with the present
invention, if the detector is disposed above the deflector, the
mirror electron is scanned on the detector by the influence of the
deflector. Therefore it is preferable to dispose the detector
between the deflector and the objective lens in measuring a
potential in accordance with the present invention.
[0052] Preferred embodiments in accordance with the present
invention will be described below referring to the drawings.
[0053] FIG. 1 is an explanatory drawing of the outline of a
scanning electron microscope. Although the explanation below is
made with an example of a scanning electron microscope (SEM)
wherein an electron beam is scanned on a sample, the application is
by no means limited to it but possibly to other charged particle
beam device as well such as a FIB (Focused Ion Beam) device, or the
like. However, according to the polarity of the charge of the beam,
it is necessary to vary the polarity of the voltage applied to the
sample. In addition, FIG. 1 explains only one embodiment of a
scanned electron microscope, and the present invention can be
applied to the scanned electron microscope with configuration other
than that of FIG. 1 in a range within the scope thereof.
[0054] In a scanning electron microscope explained in FIG. 1,
extraction voltage is applied between the field emission negative
electrode 11 and the extraction electrode 12, and the primary
electron beam is extracted.
[0055] The primary electron beam 1 thus extracted is accelerated by
the acceleration electrode 13, and is subjected to converging by
the condenser lens 14 and scanning deflection by the upper scanning
deflector 21 and the lower scanning deflector 22. The deflection
intensity of the upper scanning deflector 21 and the lower scanning
deflector 22 has been adjusted to allow two-dimensionally scanning
on the sample 23 with the lens center of the objective lens 17 as a
fulcrum.
[0056] The primary electron beam 1 deflected is further subjected
to acceleration by rear stage accelerating voltage 19 in the
acceleration cylinder 18 disposed in the passage of the objective
lens 17. The primary electron beam 1 rear stage accelerated is
converged by lens action of the objective lens 17. The cylindrical
electrode 20 is grounded and forms an electric field between the
acceleration cylinder 18 for accelerating the primary electron beam
1.
[0057] The electron such as the secondary electron emitted from the
sample or the backscatter electron is accelerated in the direction
reverse to the irradiation direction of the primary electron beam 1
by the negative voltage (hereafter referred also to as retarding
voltage) applied to the sample and by the electric field formed in
the gap with the acceleration cylinder 18, and is detected by the
detector 29.
[0058] The electron detected by the detector 29 is synchronized
with the scanning signal supplied to the scanning deflector and is
displayed on an image display device not shown. Also, the image
obtained is stored in a frame memory not shown. Further, the
current or the voltage supplied or applied to each constituting
element of the scanning electron microscope shown in FIG. 1 may be
controlled by a control device arranged separate from the main body
of the scanning electron microscope.
First Embodiment
[0059] A method for measuring a potential of a sample using an
electron beam will be described below.
[0060] A flowchart of the present embodiment is shown in FIG. 6.
Also, an outline of a charging control device is shown in FIG.
8.
[0061] In the step S1, judgment is made whether the reference
function FR of the acquisition condition to be compensated this
time has been stored or not in the reference function record part
102. If there is no reference data required for the compensation
this time in the record part 102, the reference sample or the
uncharged sample is made a mirror state in the step S100 in the
loop 1 with the condition stored in the acquisition condition
record part 103 being set, and the displacement amount or the
magnification against V.sub.r is detected by a feature amount
arithmetic unit 101 in the step S120. The reference function FR
obtainable by function fitting using the obtained displacement
amount or the magnification is obtained in the step S130, and is
stored in the reference function record part 102 in the step S140.
When the reference function FR has been obtained in the loop 1 or
there already is the reference function FR in the step 1, the
acquisition condition is read out from the acquisition condition
record part 103 by the step S100 of the loop 2 after charging of
the sample, and the mirror state is set. In the step S110, the
displacement amount or the magnification is detected against
V.sub.r by a plurality of numbers using the feature amount
arithmetic unit 101. In the step S130, the potential of the sample
V.sub.s is derived from the feature amount and the number of
references FM obtained by the potential arithmetic unit 104. In the
step S150, the compensated value of the exciting current I.sub.obj
is calculated based on the potential of the sample obtained using
the focus current control device 105, and the exciting amount of
the objective lens is adjusted. According to the present invention,
the focus control can be performed by measuring the potential of
the charged sample by the non-contact electron beam and
compensating the exciting current. With this configuration, the
focus control in observing an insulated sample can be performed in
a short time and without variation in the sample condition.
[0062] Though the present embodiment is to derive the potential of
the sample using the relation between the retarding potential
V.sub.r and the displacement amount or the magnification and to
perform the focus control by adjusting the exciting current
I.sub.obj, even if the optical parameters (retarding potential
V.sub.r and the exciting current I.sub.obj) shown above are
replaced with other optical parameters, similar effect is
expectable.
Second Embodiment
[0063] A flowchart of the second embodiment is shown in FIG. 7.
Also, an outline of a charging control device is shown in FIG.
8.
[0064] In the step S1, judgment is made whether the reference
function FR of the acquisition condition to be compensated this
time has been stored or not in the reference function record part
102. If there is no reference data required for the compensation
this time in the record part 102, the reference sample or the
uncharged sample is made a mirror state in the step S100 in the
loop 1 with the condition stored in the acquisition condition
record part 103 being set, and the displacement amount or the
magnification against V.sub.r is detected by a feature amount
arithmetic unit 101 in the step S120. The reference function FR
obtainable by function fitting using the obtained displacement
amount or the magnification is obtained in the step S130, and is
stored in the reference function record part 102 in the step S140.
When the reference function FR has been obtained in the loop 1 or
there already is the reference function FR in the step 1, the
acquisition condition is read out from the acquisition condition
record part 103 by the step S100 of the loop 2 after charging of
the sample, and the mirror state is set. In the step S110, the
displacement amount or the magnification is detected against
V.sub.r by a plurality of numbers using the feature amount
arithmetic unit 101. In the step S130, the potential of the sample
V.sub.s is derived from the feature amount and the number of
references FM obtained by the potential arithmetic unit 104. In the
step S160, the compensated value of the deflection current
I.sub.scan is calculated based on the potential of the sample
obtained using the deflection current control device 105, and the
deflection amount is adjusted. According to the present embodiment,
the magnification control can be performed by measuring the
potential of the charged sample by the non-contact electron beam
and compensating the exciting current.
[0065] Though the present embodiment is to derive the potential of
the sample using the relation between the retarding potential
V.sub.r and the displacement amount or the magnification and to
perform the magnification control by adjusting the deflection
current I.sub.scan, even if the optical parameters (retarding
potential V.sub.r) shown above are replaced with other optical
parameters, similar effect is expectable.
[0066] In addition, feedback to the magnification of the obtained
image may be performed.
[0067] It should be understood by those skilled in the art that
various modifications, combinations, sub-combinations and
alterations may occur depending on design requirements and other
factors insofar as they are within the scope of the appended claims
or the equivalents thereof.
* * * * *